Commercial power converters are required to meet various electromagnetic interference (EMI) emission standards. EMI emission includes common mode noise. A power converter may generate common mode noise from semiconductor switching operations. For example, a flyback converter may include a primary switch coupled in series with a primary winding of a transformer, and a secondary switch (e.g., a diode or synchronous rectifier) coupled in series with a secondary winding of the transformer. The primary switch may be switched on/off to regulate an output voltage of the flyback converter, thus generating voltage transients (dV/dt) across the primary and secondary switches. These switches may be attached to heat sinks that may be further coupled to a chassis that may also serve as a ground connection for the circuit. These connections, as well as parasitic capacitances of various circuit components, may create a path from the switches to ground. The voltage transients, acting on the parasitic capacitances, may generate common mode currents iCM=C×dV/dt, where C represents the parasitic capacitance, which may cause magnetic flux and common mode noise.
When a transformer is employed in a power converter, e.g., in a flyback converter, the common mode current generated in the primary side may flow to the secondary side through inter-winding capacitance between the transformer's primary and secondary windings, and vice-versa. Thus, one way to address the common mode noise issue is to add an inter-winding shield (e.g., an electrostatic screen) in a transformer to block transmission of the currents. For example, a shield layer may be inserted between the primary and secondary winding. For safety considerations, the shield layer may be connected to a primary return terminal (PRTN), i.e., ground at the primary side, thus recirculating the common mode current induced by the primary switch back to the PRTN. However, currents may still be induced at the secondary side and flow to the primary side, e.g., through the shield layer. Due to fabrication limits, it may be difficult to include addition of more shield layers in the transformer that are connected also to the secondary return (SRTN), i.e., ground at the secondary side. In other words, common mode currents generated at the secondary side may still flow back to the PRTN. Thus, what is needed is an approach to mitigate common mode noise, including the inducement of common mode noise from the secondary side to the primary side.